JP,B 4-34686, for example, discloses one of prior-art heating resistor flow rate measuring instruments.
Also, JP,A 8-159833, for example, discloses one of heating resistor flow rate measuring instruments capable of measuring the flow rate of a fluid when the fluid flowing in the forward direction may cause a pulsation accompanying a backward flow.
In the flow rate measuring instrument disclosed in JP,A 8-159833, a sub-passage is provided in a main passage, and a heating resistor is disposed in the sub-passage. The sub-passage includes a pair of sub-passage portions that are extended substantially parallel to a center axis of the main passage and are opened in directions opposite to each other. An end of one sub-passage portion positioned away from the opening side has a communicating portion communicated with the vicinity of an opening of the other sub-passage portion. The heating resistor is disposed in each sub-passage between the two communication portions.